1. Field of the Invention
The present invention relates generally to a detector for detecting a microwave transmitted from a measuring equipment, such as a speed measuring radar or so forth. More specifically, the invention relates to a multi-band microwave detector which is capable of discriminating a microwave from a specific source from those from other sources.
2. Description of the Related Art
Multi-band microwave detectors, such as so-called radar detectors which are adapted to detect microwaves transmitted from radar type speed measurement equipments for alarming, have been known in the prior art. For example, U.S. Pat. No. 4,622,553, issued on November 1986 to Baba et al., U.S. Pat. No. 4,630,054, issued on Dec. 16, 1986 to Martinson, U.S. Pat. No. 4,954,828, issued on Sep. 4, 1990 to Orr disclose radar detectors of the type that the present invention is directed. In case of typical traffic monitoring radar type speed measuring equipment (hereinafter referred to as "radar") employs a microwave in a plurality of frequency bands, i.e. 10 GHz band (X band), 24 GHz band (K band) and 35 GHz band (Ka band). The known radar detectors receive the microwaves transmitted from the radar through a heterodyne type receiver circuit, such as a super-heterodyne receiver circuit or so forth. For widening receiving band range, a local oscillation frequency is swept. In addition, for assuring detection, sweeping of the band is temporarily stopped for a given period at every occurrence of reception of signal. In the known method of discrimination of the reception signal, a signal waveform of heterodyne reception characteristics within a signal detection period where sweeping is stopped, is divided into a period, in which the magnitude of signal is held higher than or equal to a given value and a period, in which the magnitude of the signal is held lower than the given value for performing discrimination on the basis of ratio of occurrence of respective periods, occurring positions or number of occurrence within reception period for respective bands. Since two frequencies are selected from a plurality of higher-order high harmonics including a dominant wave simultaneously transmitted from a single local oscillator for X band and K band local oscillation frequencies and used simultaneously for a frequency mixing, the sweeping of the band is performed adapting to either of X band or K band having wider target frequency band. On the other hand, another local oscillator is employed for Ka band local oscillation frequency. In detection, the frequency from the local oscillator for the X and K band local oscillation frequencies and the frequency from the local oscillator for the Ka band local oscillation frequency are selectively used. Namely, in the known microwave detector, the local oscillator for receiving means for X and K band, which is not switched in time series and the local oscillator for the Ka band are employed for switching in time sharing manner.
When the microwave in the targeted band is detected by the microwave detector set forth above, it is still not certain whether the received microwave is transmitted from a specific source to be detected or from other source. This fact serves as a limit of reliability of this type of microwave detector in practical use.
In general, the microwave detector has a mixer portion exposed in the vicinity of a throat opening portion of a horn antenna and a local oscillator for generating a microwave for frequency mixing within a shield box which is located inner side to the mixing portion. In the reason of construction set forth above, the microwave frequency is inherently emitted to a space and then received by the mixing portion. Therefore, it is inevitable to cause leakage of the microwave containing not only the dominant frequency but also higher-order high harmonic frequencies through the throat opening portion of the horn antenna. When a plurality of microwave detectors are used in relatively close proximity to each other, the microwave leaked from one microwave detector may be received by another microwave detector to cause erroneous alarm despite of the fact that the source of the received microwave is different from the targeted source. Such problem can be caused by the combination of the leaking microwave frequency from the local oscillator and the receiving band.
In particular, when the leaking microwave frequency is completely included in the frequency band of the microwave from the targeted source, it is not possible to discriminate such leaking microwave from another microwave detector from that from the targeted source, e.g. the radar by the conventional discrimination method, in which the signal processing based on the heterodyne reception characteristics is performed. Therefore, due to impossibility of discrimination between the microwaves from the targeted source and from another microwave detector, it is inherent to generate an alarm even when the microwave from another microwave detector is received. Otherwise, the microwave detector may not achieve the desired function. Furthermore, it is completely impossible in the prior art to avoid erroneous alarming due to the presence of the leaking microwave from another microwave detector when both microwaves from the targeted source and another microwave detector are present.